1. Field of the Invention
The present invention relates to testing or measuring the size of an inner diameter of a bore and, more particularly, to an assembly for use with an air gauge to test or measure a diameter of a rifled barrel of a firearm.
2. Prior Art
All rifled barrel firearms have a series of internal lands and grooves on a helix angle that cause the bullet to spin about it's axis thereby providing flight stability. The lands make up the bore diameter and the grooves form the rifling diameter. The helix is also commonly called twist. Bore and rifling diameters can be measured throughout the entire length of the barrel with a very accurate gauging method using an air gauge. All air gauges have a gauging shank that is just small enough to fit inside the bore to be measured. Two or more air holes (orifices) strategically located on the gauging shank emit air at approximately 40 psi to the inside surface of the bore being measured. The resistance to air flow, or back pressure, is transmitted to a liquid column or electronic differential air pressure measuring device that is calibrated in thousandths, or even ten thousandths of an inch. The amount of resistance to air flow through the orifices can easily determine the exact size of the bore diameter.
In order to accurately measure the bore diameter inside the complete length of the firearm barrel, the air orifices in the gauging shank must rotate in exact proportion to the rate of twist in the barrel rifling in order to keep the air orifice holes on the center of the lands. Generally speaking, the timed rotation of the air orifices is accomplished by use of a timing sleeve that is nothing more than a short round piece of ground carbide that has grooves machined into it at the nominal twist rate which is affixed to the gauging shank of the testing spindle. A long tube and handle is connected to the gauging shank which allows inspection of the entire barrel length. Calibrated gauging or test rings are used to accurately set the high and low limits on the air gauge scale which establish the acceptable limits of the bore being measured. The test rings are slipped over the gauging shank covering the air orifices, thus creating back pressure of a known size. The part being measured should be within the upper and lower limits set on the air gauge scale.
Presently available bore air spindles generally have a gauging shank, air orifices, and a timing sleeve. The spindles are attached to a long tube and handle that can be connected to the air pressure measuring device. Generally, there are two types of bore air spindles known in the prior art. One type of spindle is a solid one piece member where all of the necessary features have been machined into it. This first type of spindle is usually made of solid carbide. The second type of spindle known in the prior art is a two piece assembly comprised of a steel gauging shank and a carbine timing sleeve permanently brazed in position on the gauging shank. Both of these prior art spindles have the tube and handle permanently brazed onto the end of the gauging shank.
During usage of the prior art spindles, the air spindle is inserted into the barrel bore and it rotates with the rifling helix (twist) throughout the entire length of the barrel. The inspector using the air gauge carefully monitors the calibrated gauge scale for any deviations that may exceed the tolerance. After hundreds of such inspections, the carbide timing sleeve will begin to wear because of the abrasive nature of barrel steel against the spindle. Some wear occurs on the outside diameter of the carbide sleeve, but it is minimal and doesn't significantly affect function or gauge accuracy. Another wear area is the outside diameter of the gauge shank in the area where it comes in contact with the lands. The most critical wear area is on the sides of the carbide grooves where the barrel lands engage the carbide sleeve grooves.
As the carbide grooves wear they become wider. Wider grooves allow the entire air spindle to move radially from one side to the other. In other words, the gauge becomes sloppy as the fit of the carbide sleeve in the rifled bore becomes loose. Excessive radial play of the spindle in the barrel will allow the air orifice of the spindle to move from the center of the rifling lands towards the edge of the lands which allows air to spill off of the lands into the grooves resulting in erroneous gauge readings. There is no practicable or economical way of repairing the prior art one or two piece spindles presently used because the combination of all three types of wear discussed above renders the prior art spindles useless and total replacement is necessary.
It is therefore an objective of the present invention to provide a new and improved spindle assembly that overcomes disadvantages in the prior art as well as provide additional features.